D．what　[参见上题解析。and前的when也是提示，故答案为A。]　[我们人类也应该“学会”这一点：有时接受就是一阅读理解beyond the earth.Hundreds of biologists，geologists，chemists and astronomers have conducted research on astrobiology(天体生物学)．That’s because there is plenty to think about before we actually find alien life.Scientists tend to be more confident than ever that life does exist out in the universe.

　　In 1996 Onstott dug up rocks a mile below the surface，and he extracted (提取) bacteria that were living cheerfully in harsh conditions completely isolated from the rest of the biosphere (生物圈)．Onstott’s discovery is just one of dozens that established the existesce of so­called extremophiles(嗜极生物)，bacteria and other forms of life thriving in such hostile places.Life can deal with a wider range of conditions.

　　Miller had probed the origin of life by passing electricity through a glass container of organic chemicals to see what came out.Bada found fascinating hints that the origin of life may well be on an Earth­like planet rather than the exception.Kaufman dealt with the question of how many such planets are likely to exist.

　　Unfortunately，the most habitable planet found is widely believed not to exist life at all.Butler and Vogt appropriately noted that their find would have to be confirmed by others same applies to evidence of alien life.In no case was any real E．T.proved to exist.The twin Viking probes landed on Mars and performed on­site tests of the soil，looking for life.Most came back negatively，but one，designed by NASA scientist Gilbert Levin，showed suspicions activity.

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